Chrome-tanning mixtures and preparation of leather therewith



United States Patent 9 2,997,364 CHROME-TANNING MIXTURES AND PREPARA- TION F LEATHER THEREWITH Ernst Komarek, Leverkusen, and Gustav Mauthe, Op-

laden, Germany, assignors to Farbenfabriken Bayer Aktieugesellschaft, Leverkusen, Germany, a corporation of Germany N0 Drawing.' Filed Dec. 2, 1957, Ser. No. 699,934 Claims priority, application Germany Dec. 11, 1956 6 Claims. (Cl. 8-94.26)

In the manufacture of vegetable or chrome-vegetable leathers the leather colour is of great importance, particularly when using inexpensive, dark-tanning tanning agents such as chestnut, myrabolam, valonia or mangrove. The colour of the leather is of particular importance in the combined chrome-vegetable tanning process. In addition to the dark colour of the vegetable tannins, the green colour of the chrome tannins has in this case a detrimental efiect. mixed shade of the leathers thus obtained considerably reduces their values.

It is now customary in vegetable tanning to improve the colour of the leather by pre-treating the hides with suitable bright-tanning synthetic tanning agents as well as by the additional use of synthetic tannins. Numerous synthetic tanning agents of various constitution have been successfully used on account of their particular properties as preliminary tanning agents.

It has now been found that surprisingly light coloured leathers are obtainable by treating the hides with tanning agents formed by mixing normal or basic trivalent chrome salts (such as potassium chromium sulphate, Hexaquo chromium sulphate, and generally other commercial chromium sulphate tanning solutions; furthermore such self-reduced chromium tanning solutions which are obtained by known reduction of bichromates for tanning purposes) with such products as are produced by known methods:

(a') from phenols by condensation with sulphites and/ or bisulphites and aldehydes and/ or ketones (see e.g. such condensation products described in U.S. Patents No. 2,320,678; No. 2,621,164; No. 2,677,675; No. 2,694,- 051), or

(b) from methylol compounds of phenols by condensation with sulphites and/or bisulphites (see e.g. such condensation products described in British Patent No. 701,103 and in French Patent No. 1,023,645), if desited in the presence of alcohols or aldehydes and/or ketones,

and by finishing the tanning with vegetable and/or synthetic tanning agents (e.g. vegetable tanning agents such as mimosa, quebracho, chestnut, myrabolam, and the like, or synthetic tanning agents such as those described in U.S. Patents No. 2,204,512; No. 2,230,564; No. 2,204,- 512; No. 2,179,038; No. 2,045,049; British Patents No. 510,743; No. 615,190; No. 683,084; No. 595,337; and French Patent No. 905,003). In contrast to this, preliminary tannages with corresponding combinations of trivalent chromium salts with other synthetic tanning agents, particularly those whose solubilizing group is a sulpho group directly attached to the aromatic nucleus, do not yield any or only a very slight brightening and levelling of the leather shade. The influence of such products on the colour of the leather is in this case not greater'than the effect obtained with such sulphonated synthetic tanning agents when used alone without the addition of the chromium component. The eifect of the products described above from mixtures of trivalent chromium salts with synthetic tanning agents obtained by condensation with sulphites, is probably partly due to the saturation of the leather surface with complex com The olive-tinted dirty K 2,997,364 Patented Aug. 22, 1961 pounds formed during the combining of the two components. In the vegetable final tanning, the hides prepared by preliminary treatment with these products do not offer any possibility of attack to the dark, in most cases also very astringent, tannin components.

The extraordinary influence of the new products on the hide surface also manifests itself in a grain-resistance which becomes particularly resistant to wrinkled grain and milled grain (mechanical strain on the grain). On account of these useful influences of the preliminary treatment, it is possible to finish the vegetable tanning immediately in concentrated tanning liquors which results in a substantial acceleration of the tanning processand thus in a highly economical production. I

The particular properties of the leathers pre-tanned with the aforesaid products are not due, as has been verified, to the individual eflects of the two components.

They are neither to be explained, as experiments have established by the acid present in the trivalent chromium salts. In this case, a specific effect of the complex compounds formed by the combining of the components is rather the reason. Thus, if the preliminary treatment is successively carried out with the individualcomponents of the mixture of the invention, the vegetable final tan-,

ning yields leather which does not show any or only a slight improvement in colour and level of colour. In

contrast to this, when using the two components of the mixture at the same time and in quantities similar to those used in the control tests, there is obtained outstandingly bright leather having an extremely uniform colour. Un-

even grain and faulty grain are compensated to a large extent.

The effect of the new products is determined to a great extent by the self-tanning capability of the synthetic component. phonic acid as a monomolecular sulpho-methylation product does not show a self-tanning capability and isaqueous solutions depends on the properties of the two' components. With chromium salts, the content of neutral salts and the basicity have a decisive influence, the tolerance and stability in aqueous solutions decreasing with increasing content of neutral salts and increasing basicity. The synthetic component of the products according to the invention involves a decrease in tolerance and stability with increasing particle size. 7 The new products can be usefully applied to the aftertreatment of vegetable-tanned leather or leather Which-- has been pretanned with a synthetic tanning agent. -In

addition to abrightening and levelling of the leather" shade, this aftertreatment involves a fixation of-the bound tanning agent and also an improvement of the resistance of the leathers to perspiration.

The following examples are given for the purpose-of illustrating the invention.

EXAMPLE 1 parts by weight of cow hide are pre-treated'; after the lime house operations usual for sole leathers (liming, deliming, pickling), in a drum with 250 parts of water and 3 parts by weight of the product described below.

The product is added in 3 portions at intervals of /2 hour each. The final tanning is carried out with a vegetable] tannin mixture of the following composition:

60 parts of pure tannin chestnut, normal 20 parts of pure tannin mimosa For example, hydroxyphenyl-methane-sul-' 10 parts of pure tannin valex 10 parts of pure tannin myrabola-m (a) In the colour pit-50 parts by weight of the pretanned unrinsed hide are introduced into a colour pit. The first (weakest) colour consists of a tan liquor 6 B. of the above mentioned composition.

-(b) In the drum.-50 parts by weight of the pretanned hide are finally tanned at a length of liquor of 400 percent referred to the hide weight, with 35 percent pure tannin of the above mentioned mixture. One half of the vegetable tanning agent is introduced first and the remain ing in three portions at intervals of 2 hours. The hides are fully tanned after three days of milling.

The leather is briefly soaked, de-oiled and dried as usual. Parallel with these finishing tannings, control tests were effected with the same hide material, but without the preliminary tanning according to the invention.

The pre-treated leather distinguished itself by an outstandingly bright leather shade and even grain. The corresponding control tests showed, in addition to the dark uneven leather shade, a strongly drawn grain or milled grain.

Composition of the product used for the preliminary treatment 400 parts by weight of a mixture of phenol and cresol (1:4) are condensed in the presence of 20 parts by weight of a 40 percent sodium hydroxide solution with 490 parts by volume of 30 percent formaldehyde at 65-70 C. for 3 hours. After the addition of 425 parts by weight of a 40 percent sodium bisulphite solution and 10 parts by weight of methanol, the mixture is heated to 95 C. and kept at this temperature, until dissolved. The product is cooled to 80 C. and after-treated wth 30 parts by weight of urea.

450 parts by weight of the condensation product thus obtained are mixed with a solution of 300 parts by weight of chrome alum in 300 parts by weight of hot water and dried by spraying.

EXAMPLE 2 100 parts by weight of cow hide are suspended, after customary lime house operations for sole leather, in a pit containing a solution of the preliminary tanning agent at 1.5 B. described in Example 1. After 24 hours (with very thick material after about 36 hours), the new product has uniformly struck through the hide which can be seen from the blue colour of the cut. With continuous use of the preliminary tanning liquor, a stationary equilibrium takes place at a strengthening of the preliminary tanning liquor with 6 percent calculated on the hide used. The unrinsed hides coming from the preliminary treatment are introduced into a solution of normal chestnut at 6 B., and, eventually finally tanned in a solution of the same tanning agent at 10 B6. The fully tanned leather is soaked, de-oiled and dried as usual.

In contrast to "an untreated control test, these chromevegetable-tanned leathers also show all the advantages described in Example 1. Another advantage of the semichrome sole leather produced according to this process consists in that it may be used in all cases where lack of drums prevails or drum tannage cannot be eifected for other reasons. Moreover, this process saves operations such as pickling and neutralization in the chrome tanna-ge which usually precedes the vegetable tanning. The pretreated leather distinguishes itself by its great flexibility.

EXAMPLE 3 100 parts by weight of thin vachette skin suitable for the manufacture of bag leather are pre-tanned, after the usual preparation in the lime house, in a milling drum with 250 parts by weight of water and 5 parts by weight of the condensation product described below. The product is added in 3 portions at intervals of /2 hour each time, and milled after the last addition for another 2 hours.

The hides thus pre-treated and briefly rinsed are introduced in a colour pit containing a tanning agent of the following composition:

30 parts of pure tannin chestnut normal 30 parts of pure tannin mimosa 30 parts of pure tannin fir bark extract l0 parts of pure tannin quebracho The concentration of the weakest colour was 4 B., that of the best (strongest) 12 B.

The leather rinsed, fat-liquored and dried in usual manner are compared with a corresponding control test without preliminary treatment. The pro-treated leathers distinquish themselves by an extraordinarily bright and uniform colour and a good grain quality.

Composition of the condensation product used for the preliminary treatment parts by weight of sheep skin prepared as usual in the lime house, are milled in a drum with 200 parts of water and 5 parts by weight of the product described below for 3 hours. A concentrated solution of 15 parts by weight of sulphited quebracho which is soluble in the cold is introduced into the same liquor and the tanning of the skins is finished overnight. Sheep lining leather rinsed and fiat-liquored as usual distinguishes itself by a bright shade of the leather without showing the reddish tint which is often objected to in the case of leather tanned with quebracho. Moreover, the pro-treated leather exhibits a better plumpness and tensile strength.

Composition 0 the product used for the above preliminary tonnage 188 parts by weight of phenol are treated with 100 parts by Weight of 30 percent formaldehyde and 2.5 parts by weight of concentrated hydrochloric acid and heated under reflux for 2 hours. Into the low molecular weight resin thus formed, a solution of 100 parts by weight of sodium bisulphite, 20 parts by weight of sodium hydroxide, 100 parts by weight of 30 percent formaldehyde and 75 parts by weight of water is introduced, and the reaction mixture heated under reflux for 6 hours. After cooling, it is neutralized with a strong acid. 200 parts by weight of the condensation product thus formed are mixed with a solution of 15 parts by weight of chromium sulphate in 15 parts by weight of water.

EXAMPLE 5 1200 parts by weight of a sammed inside sole leather are treated with 700 parts of water and 60 parts by weight of the product described in Example 1 and milled in a drum for 3 hours. The leather thus after-treated is briefly soaked, de-oiled and dried. Compared with untreated leather, the treated inside sole leather shows a bright uniform colour. At the same time, the free tanning agent is strongly fixed and the resistance to perspiration is considerably increased.

We claim:

1. A process for the manufacture of leather which comprises pretannin-g a hide with a mixture of a trivalent chromium salt and a synthetic tanning agent prepared by the condensation under alkaline conditions of an unsulfonated phenol with (a) a member selected from the group consisting of sulfites and bisulfites, and (b) a further member of the group consisting of 'aldehydes and ketones, the chromium tanning salt and the synthetic tanning agent being employed in about equal amounts, and then retanning the pretanned hide with a member selected from the group consisting of vegetable tanning agents and synthetic tanning agents, the latter being reaction products of low molecular Weight sulfomethylated phenols, formaldehyde, and sulfites or bisulfites.

2. A process for the manufacture of leather which comprises pretanning a hide With a mixture of a trivalent chromium tanning salt "and a synthetic tanning agent prepared by condensation of an unsulfonated methylolated phenol with a member of the group consisting of a sulfite and a bisulfite in the presence of a member selected from the group consisting of an alcohol, analdehyde, and a ketone under alkaline conditions.

3. Process for the manufacture of leather which comprises after-treating a member selected from the group consisting of a vegetable-tanned leather and a leather tanned with a synthetic tanning agent, with a mixture of a trivalent chromium salt and a synthetic tanning agent prepared by the condensation under alkaline conditions of an unsulfonated phenol with a member selected from the group consisting of sulfites and bisul-fitcs, and a further member of the group consisting of aldehydes and ketones, the chromium tanning salt and the synthetic tanning agent being employed in about equal amounts.

4. Process for the manufacture of leather which comprises after-treating a member selected from the group consisting of a vegetable-tanned leather and a leather tanned with a synthetic tanning agent, with a mixture of a trivalent chromium tanning salt and a synthetic tanning agent prepared by condensation of an unsulfonated methylolated phenol with a member of the group consisting of a sulfite and a bisulfite in the presence of a member selected from the group consisting of an alcohol, an aldehyde, and a ketone, under alkaline conditions.

5. A tanning mixture of approximately equal parts of a trivalent chromium tanning salt and a synthetic tanning agent prepared by the condensation of an unsulfonated phenol with (a) a member selected from the group consisting of sulfites and bisulfites, and (b) a further member of the group consisting of aldehydes and ketones, under alkaline conditions.

6. A tanning mixture of approximately equal parts of a trivalent chromium tanning salt and a synthetic tanning agent prepared by condensation of an unsulfonated methylolated phenol with a member of the group consisting of a sulfite and a bisulfite in the presence of a member selected from the group consisting of an alcohol, an aldehyde, and a ketone under alkaline conditions.

Rev. Tech. des Industries du Cuir, 43 :9 September 1951 pp. -186 of an article appearing in Leder-und- Hautemarkt, 1951, No. 31, May 8, 1951, 32.

Gustavson: Chemistry of Tanning Processes, Academic Press, New York, N.Y., 1956, pp. 237 and 308. 

2. A PROCESS FOR THE MANUFACTURE OF LEATHER WHICH COMPRISES PRETANNING A HIDE WITH A MIXTURE OF A TRIVALENT CHROMIUM TANNING SALT AND A SYNTHETIC TANNING AGENT PREPARED BY CONDENSATION OF AN UNSULFONATED METHYLOLATED PHENOL WITH A MEMBER OF THE GROUP CONSISTING OF A SULFITE AND A BISULFITE IN THE PRESENCE OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF AN ALCOHOL, ANALDEHYDE, AND A KETONE UNDER ALKALINE CONDITIONS. 